1. FIELD OF THE INVENTION
The present invention relates to a process for polymerizing .alpha.-olefins in a solvent under the condition that the polymer formed grows as particles, in which the process comprises reusing a greater part of the solvent used for the polymerization by recycling the solvent to the polymerization zone without carrying out any purification treatment. In greater detail, the present invention relates to a process which comprises carrying out a slurry polymerization reaction of .alpha.-olefins in the presence of a Ziegler-Natta catalyst, and particularly a polymerization reaction of ethylene and propylene, using an inert hydrocarbon liquid or a liquefied monomer to be polymerized as a solvent or as a dispersing agent (hereinafter, it is called a solvent), wherein said process is characterized by that the solvent and the produced polymer are separated before adding a polymerization inhibitor or a catalyst decomposing agent to the polymerization reaction product, and the separated solvent is reused by recycling to the polymerization reactor.
2. DESCRIPTION OF THE PRIOR ART
The polymerization of .alpha.-olefins in the presence of a Ziegler-Natta catalyst is generally carried out in an inert hydrocarbon liquid or a liquefied monomer. The temperature at which the polymer formed exists in the liquid as solid particles is usually used in the reaction. As is well known, since the catalysts used in such a polymerization are generally compound systems consisting mainly of a compound of a transition metal, such as titanium and vanadium, and an organic compound of a metal, such as aluminium, the polymers produced contain these metals. Accordingly, in such a polymerization, an important post treatment of the polymer is the removal of the catalyst from the polymer by dissolution and extraction simultaneously with the stopping of the polymerization reaction.
The stopping of the polymerization reaction and the solubilization and extraction of the catalyst are usually carried out in the form of a solvent slurry, because the polymer formed is a solid. It is operationally advantageous that this solvent be that which is used in the polymerization. However, since the polymerization inhibitor and the solubilizing agent (usually alcohols are used for both purposes) added in the step of treating the catalyst are, as a matter of course, catalyst poisons and the decomposition product formed by the treatment of the catalyst are also metal compounds, it is not practical to utilize the solvent containing such metals for polymerization by recycling the solvent to the polymerization zone as it is. Therefore, it is impossible to use the same solvent for the polymerization by recycling the solvent if the solvent is not purified using a distillation, etc. Thus, this is an important disadvantage of the slurry polymerization process utilizing a solvent from the standpoint of cost.